1. Field of the Invention
The present invention relates to means and a method for indicating the curing progress in curable compositions, particularly in dual component polyurethane systems, and the use of dyes containing quinoniod groups as color indicators in compositions prepared from isocyanates.
2. Description of the Related Art
Polymers that are obtained by polyaddition of dihydric or polyhydric alcohols and isocyanates are referred to as polyurethanes (PUR). Conventional polyurethanes are produced from polyester dials and polyether diols and diisocyanates such as toluene diisocyanate (TDI), 4,4′-methylene diphenyl isocyanate (MDI) or hexamethylene diisocyanate (HMDI). The use of dials and isocyanates in equimolar ratios leads to linear polyurethanes. Branched and cross-linked products are produced when higher-functionality educts are used as supplements to the polyurethane synthesis or also when there is an excess of isocyanate, in which case isocyanate groups react with urethane groups or urea groups. Certain amines and organic tin and zirconium compounds are used as catalysts for the polyaddition. Usually, polyurethanes are produced as prepolymers having terminal isocyanate groups, which, when used, for example, as sealing compositions, and are cured under the action of moisture from the atmosphere, leading to chain extension and, possibly, to crosslinking. Such prepolymers can also be chain-extended with diamines to produce polyurethanes containing urea groups.
Polyurethane foams are formed when the polyaddition is carried out in the presence of water or carboxylic acids. These react with the isocyanates with the elimination of carbon dioxide, which has an expanding and foam-forming action. Polyurethane-based resins are referred to as polyurethane resins. They are produced, for example, by the reaction of monoglycerides and diglycerides with diisocyanates such as toluene diisocyanate (TDI). Polyurethane resins also include polyurethane prepolymers terminated by free or blocked isocyanate group, which are cured under the action of moisture.
Depending on the choice of starting materials selected and their stoichiometric ratio, polyurethanes can be produced with very different mechanical properties. They can be used, for example, as constituents of adhesives and coatings primarily in the form of polyurethane resins, as polyether-urethane or polyester-urethane rubber, as thermosetting casting resins, and as foamed plastics.
The term “polyurethane system” used below is understood to mean all polymers that contain chain-extending or crosslinking urethane groups derived from the reaction of isocyanates with polyols.
Usually, dual component polyurethane systems are used as coating, insulating, and sealing compositions. In these systems, one component contains the polyols and the other component, also referred to as the curing agent, contains the isocyanates. Prior to application, the two components are mixed and the mixture is then cured. The prerequisite for a uniform curing process is the thorough mixing of the two components. Furthermore, particularly polyurethane systems that are to be processed further must be fully cured prior to further processing. With conventionally used dual component polyurethane systems, it is difficult to determine when the mixture of the components is homogeneous and when the resulting composition is fully cured.
It is known to use a dye or pigment in each of the two components of a polyurethane system, which dyes or pigments initially differ from each other but on mixing form a new color. This, however, indicates exclusively the homogeneity of the mixture.
The use of pH indicators is known for monitoring the progress of curing. Thus DE 101 15 004 A1 describes a moisture-curing polyurethane system that contains, prior to moisture curing, an excess of isocyanate groups and is produced from compositions having at least two isocyanate groups, compositions having at least two hydrogen atoms capable of reacting with isocyanate groups, and an amine compound. This polyurethane system contains a pH indicator that indicates the degree of curing of the system, i.e. the increase in pH due to the increase in free amine groups during the curing process, by a change of color.
However, the use of a pH indicator is not suitable for polyurethane systems showing no significant change in pH during the curing process.